1. Field of Invention
The present invention relates to engines, and more particularly to a loop-scavenged internal combustion engine having a heat recovery steam generator exhaust manifold (“HRSG”) to produce steam for injection into the cylinders.
2. Background Art
It is generally accepted the conventional steam engine technology matured in the 1950s, and even today steam engines are criticized for low efficiency.
The low operational efficiency of the steam engine is reflected primarily by the energy required to convert water into steam, which is approximately 1000 Btu's per pound of water to make 1 pound of steam. Additionally, the Water Rate of the steam engine (pounds (lbs) of steam it takes to generate 1 horsepower) is affected by the initial steam pressure, exhaust backpressure, and whether the steam engine is of the counterflow type or uniflow type.
In a counterflow type steam engine, the inlet and exhaust passages are located in the cylinder head, and cool exhaust gasses must pass over hotter cylinder walls and the cylinder head during the exhaust stroke, thereby cooling the surfaces below the temperature of the incoming inlet steam. This causes condensation on the cylinder walls and removes lubrication. The incoming steam therefore has to reheat the head and cylinder walls back up to the inlet steam temperature before useful work can be generated.
“Initial condensation” refers to a loss of steam pressure before work occurs due to condensation of the incoming inlet steam. The cause of initial condensation is related to and a component of “wall effects” which is an industry reference to the “missing quantity” of steam between the steam consumption shown in the indicator diagram (pressure volume diagram) and direct measurement, where the theoretical amount of missing steam is less than the actual amount. It relates to the chilling of the interior passages and surfaces or “walls” of the steam engine that are swept by the exhaust steam. If the temperature of the walls is higher than the steam saturation temperature, no condensation occurs, and the heat transfer follows “gas laws,” the loss being very small. However, if the temperature of the walls is below the saturation temperature of the incoming inlet steam, condensation occurs. Condensation results in a loss of pressure, which is the driving force of the steam engine. In certain designs, particularly counterflow steam engines, where the exhaust steam retraces the inlet steam admission path, condensation is large. The ratio of expansion between the inlet and exhaust steam relates to the mean temperature of the walls in the counterflow type steam engine. For high expansion rates, superheating the admission steam raises the wall temperatures and is partially effective at minimizing wall effects. Even though waste heat is recovered (transferred back into) the exhaust steam of the counterflow type steam engine, a rise in entropy occurs leading to an irreversible loss according to the second law of thermodynamics. Decreasing the expansion ratio, or temperature difference between admission steam and exhaust steam, is also effective at reducing wall effects of a counterflow type steam engine. Two techniques that maintain a high expansion ratio, yet limit the temperature difference, are the use of multiple cylinders to expand the steam (known as compounding), the preventing the incoming inlet steam from retracing its path in the engine, which is known as a uniflow type or Unaflow type steam engine.
During steady operation of a uniflow type steam engine, the temperature of the engine surfaces exposed to incoming inlet steam stays very close to the temperature of the incoming inlet steam (suffering primarily only radiation losses) and as the cylinder walls are exposed as the piston moves, the temperature corresponds closely to the expansion of the steam in the cylinder, resulting in minimal heat loss. Steam is exhausted through exhaust ports at the termination of the stroke. The uniflow type engine is currently the best known principle for reducing irreversible thermodynamic losses in steam engines. The uniflow principle, along with compounding, is exemplified in contemporary multistage steam turbines. In uniflow type steam engines, the surfaces exposed to inlet steam are reservoirs, or interconnected steam chests, for incoming inlet steam. During expansion of the steam admitted to the cylinder, the surfaces are heated by the reservoirs. As the piston returns to Top Dead Center (TDC), heat from compression is transferred to the reservoirs. In this way, entropy is minimized.
In a uniflow type steam engine, steam enters from the cylinder head and travels down the cylinder as it expands and cools to an exhaust port near the end of the stroke. The exhaust steam never retraces the path of incoming inlet steam. In practice, the uniflow principle yields an approximate 20% reduction in water rate over the counterflow type steam engine at the same operating conditions.
In addition to low operational efficiency, known steam engines commonly require complex valve apparatus and relatively large valves (due to low/intermediate inlet pressure) as well as a boiler, and often a condenser, a hot well, a deaerator, and many other auxiliary systems to function. These greatly increase the complexity and expense of a steam engine power system. While the uniflow principle dramatically improved the efficiency of the steam engine, steam engines were ultimately superseded by diesel engine technology. However, diesel engine technology is not without drawbacks, which include, low operational efficiency, poor combustion, the need for exhaust after-treatment, the need for complex valve apparatus and large intake valves, and the need for large capacity water-cooling systems to absorb waste heat and reduce thermal loading.
One noted problem with diesel internal combustion engines is their low operational efficiency. This is reflected by their generation of waste of heat of combustion that is ejected into the atmosphere with exhaust gases and into the water or air cooling system (about 30% and 25% of the energy content of the fuel, respectively). The internal combustion engine is further handicapped in that combustion must occur quickly as a timed event. To increase the quality of this combustion event, high pressures (frequently in excess of 30,000 psig) are used in diesel engine fuel injectors to create a more homogeneous fuel and air mixture to improve combustion and lessen production of soot. At high specific power outputs, the combustion temperature of diesel fuel may exceed 2200F.
If the temperature and pressure are not correct, poor combustion occurs. Complex and expensive exhaust after-treatment is often used to correct the symptoms of poor combustion, but in so doing, these systems may further decrease fuel efficiency by up to 20%. Systems such as Diesel Particulate Filters (DPFs) with onboard regeneration use raw fuel to regenerate the filtering media, while Exhaust Gas Recirculation (EGR) catalytic converters and Selective Catalyst Reduction Systems (SCR) may increase exhaust backpressure. These systems further reduce fuel efficiency. EGR systems may also re-introduce soot into the cylinders, causing increased wear as well as contamination of engine oil and thereby increasing wear and tear on the engine.
In addition to low operational efficiency, internal combustion engines typically require complex valve apparatus and large intake valves because the inlet pressure is low.
Moreover, often a water cooling system is required to absorb waste heat, greatly adding to the expense of the engine system, increasing weight and increasing complexity, because demands to carry water must be created within the engine during manufacture.
Similar to what happened to steam engine technology, conventional internal combustion engine technology has also reached a mature state of development where only small incremental improvements are achieved at great expense. Like the steam engine, the internal combustion engine faces continually increasing expectations of performance, efficiency, and emissions quality, yet an improvement in one desirable feature is often made at the expense of another.
There is a need to optimize the internal combustion engine and the steam cycle to be thermally uniflow, to maximize the efficiency of the steam cycle and to capture other desirable elements of the steam engine, while reducing complexities and expense.
There is also a need to avoid large low pressure valves and valve apparatus.
There is a need to eliminate soot formation associated with known two-stroke engines.
There is a need for an engine that requires less outside energy and requires less combustion in the cylinder per brake horsepower, thereby decreasing the products of combustion that may be present in the exhaust and require after-treatment systems.
Forced-flow steam generators are used to generate steam rapidly, safely, and economically. In heat recovery applications, layers of spiral-wound coils are used, and while effective at providing a highly efficient heating surface, this arrangement requires a prohibitively large structure in order to define the necessary volume for operation, and the large structure must be mounted on a framework that is generally distal from the engine, which decreases the amount and quality of heat available for recovery.
The concept of supercritical steam generation can be traced to Mark Benson, who in 1922 patented a process for the generation of working steam ready for use at any desired pressure. Benson's objective was to generate steam that avoids evaporation. Benson believed that at less than critical pressure, bubbles of steam would occupy so much of a boiler tube's surface that the bubbles would slow down the transmission of heat between the tube wall and the water. According to Benson operation at critical pressure provides the highest available heat transfer efficiency. The Benson boiler's output is throttled through a pressure-reducing valve (commonly referred to as an evaporator) to produce dry, subcritical steam.
There is a need to optimize steam generation such that supercritical steam, high temperature steam, high temperature water, or greatest water/steam temperature generation can be created from the heat of a reciprocating engine's exhaust, while still providing the convenience of an easily removed and serviceable assembly in the form of an exhaust manifold, header, and/or exhaust pipe.
There is a need for a heat recovery steam generator that monitors and maintains the temperature of the output of each steam coil the same on a multi-cylinder engine, which may selectively load cylinders and may have independent heat rejection rates for each cylinder.
There is a need for an engine that requires less outside energy and requires less combustion in the cylinder per brake horsepower.
As described, some or all of the drawbacks and problems explained above, and other drawbacks and problems, may be helped or solved by my invention shown and described herein. My invention may also be used to address other problems not set out herein or which become apparent at a later time. The future may also bring to light unknown benefits that may, in the future, be appreciated from my novel invention shown and described herein.
My invention does not reside in any one of the identified features individually, but rather in the synergistic combination of all of its structures, which give rise to the functions necessarily flowing therefrom as hereinafter specified and claimed.